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Scientists Have Discovered A 1.7 Billion Years Old Nuclear Reactor

If you were hunting for alien intelligence, looking for a
surefire signature from across the Universe of their activity, you'd have a few
options. You could look for an intelligent radio broadcast, like the type
humans began emitting in the 20th century. You could look for examples of
planet-wide modifications, like human civilization displays when you view Earth
at a high-enough resolution.

You could look for artificial illumination at night, like our
cities, towns, and fisheries display, visible from space. Or, you might look
for a technological achievement, like the creation of particles like
antineutrinos in a nuclear reactor. After all, that's how we first detected
neutrinos (or antineutrinos) on Earth. But if we took that last option, we
might fool ourselves. Earth created a nuclear reactor, naturally, long before
humans ever existed.

In order to create a nuclear reactor today, the first
ingredient we need is reactor-grade fuel. Uranium, for example, comes in two
different naturally-occurring isotopes: U-238 (with 146 neutrons) and U-235
(with 143 neutrons). Changing the number of neutrons doesn't change your
element type, but does change how stable your element is. For U-235 and U-238,
they both decay via a radioactive chain reaction, but U-238 lives about six
times as long, on average.

By time you get to the present day, U-235 makes up only about
0.72% of all naturally-occurring Uranium, meaning it has to be enriched to at
least about 3% levels in order to get a sustaining fission reaction. But 1.7
billion years ago was more than two full half-lives ago for U-235. Back then,
in ancient Earth, U-235 was about 3.7% of all uranium: enough for a reaction to
occur.

In between different layers of sandstone, before you reach
the granite bedrock making up most of Earth's crust, you often find veins of
mineral deposits, rich in a particular element. Sometimes these are extremely
lucrative, like when we find gold veins underground. But sometimes, we find
other, rarer materials in there, such as uranium.

The Uranium-235 chain reaction that both leads to a nuclear
fission bomb, but also generates power inside a nuclear reactor, is powered by
neutron absorption as its first step, resulting in the production of three
additional free neutrons. E. SIEGEL, FASTFISSION / WIKIMEDIA COMMONS

In modern reactors, enriched uranium produces neutrons, and
in the presence of water, which acts like a neutron moderator, a fraction of
those neutrons will strike another U-235 nucleus, causing a fission reaction.
As the nucleus splits apart, it produces lighter daughter nuclei, releases
energy, and also produces three additional neutrons. If the conditions are
right, the reaction will trigger additional fission events, leading to a
self-sustaining reactor.

Two factors came together, 1.7 billion years ago, to create a
natural nuclear reactor. The first is that, above the bedrock layer of granite,
groundwater flows freely, and it's only a matter of geology and time before
water flows into the uranium-rich regions. Surround your uranium atoms with
water molecules, and that's a solid start.

Geologic cross-section of the Oklo and Okélobondo uranium
deposits, showing the locations of the nuclear reactors. The last reactor (#17)
is located at Bangombé, ~30 km southeast of Oklo. The nuclear reactors are
found in the FA sandstone layer. MOSSMAN ET AL., 2008; REVIEWS IN ENGINEERING
GEOLOGY, VOL. 19: 1-13

But to get your reactor working well, in a self-sustaining
fashion, you need an extra component: you want the uranium atoms to be
dissolved in the water. In order for uranium to be soluble in water, oxygen
must be present. Fortunately, aerobic, oxygen-using bacteria evolved in the
aftermath of the first mass extinction in Earth's recorded history: the great
oxygenation event. With oxygen in the groundwater, dissolved uranium would be
possible whenever water floods the mineral veins, and could have even created
particularly uranium-rich material.

A selection of some of the original samples from Oklo. These
materials were donated to the Vienna Natural History Museum. LUDOVIC
FERRIÈRE/NATURAL HISTORY MUSEUM

When you have a uranium fission reaction, a number of
important signatures wind up being produced.

1.Five isotopes of
the element xenon are produced as reaction products.

2.The remaining
U-235/U-238 ratio should be reduced, since only U-235 is fissile.

3.U-235, when
split apart, produces large amounts of neodymium (Nd) with a specific weight:
Nd-143. Normally, the ratio of Nd-143 to the other isotopes is about 11-12%;
seeing an enhancement indicates uranium fission.

4.Same deal for
ruthenium with a weight of 99 (Ru-99). Naturally occurring with about 12.7%
abundance, fission can increase that to about 27-30%.

In 1972, the French physicist Francis Perrin discovered atotal of 17 sites spread across three ore deposits at the Oklo mines in Gabon,
West Africa, that contained all four of these signatures. The Oklo fission
reactors are the only known examples of a natural nuclear reactor here on
Earth, but the mechanism by which they occurred lead us to believe that these
could occur in many locations, and could occur elsewhere in the Universe as
well.

This is the site of the Oklo natural nuclear reactors in
Gabon, West Africa. Deep inside the Earth, in yet unexplored regions, we might
yet find other examples of natural nuclear reactors, not to mention what might
be found on other worlds. US DEPARTMENT OF ENERGY

When groundwater inundates a uranium-rich mineral deposit,
the fission reactions, of U-235 splitting apart, can occur. The groundwater
acts as a neutron moderator, allowing (on average) more than 1 out of 3
neutrons to collide wtih a U-235 nucleus, continuing the chain reaction. As the
reaction goes on for only a short amount of time, the groundwater that
moderates the neutrons boils away, which stops the reaction altogether.

Over time, however, without fission occurring, the reactor
naturally cools down, allowing groundwater back in. By examining the
concentrations of xenon isotopes that become trapped in the mineral formations
surrounding the uranium ore deposits, humanity, like an outstanding detective,
has been able to calculate the specific timeline of the reactor. For
approximately 30 minutes, the reactor would go critical, with fission
proceeding until the water boils away.

The terrain surrounding the natural nuclear reactors in Oklo
suggests that groundwater insertion, above a layer of bedrock, may be a
necessary ingredient for rich uranium ore capable of spontaneous fission. CURTIN UNIVERSITY / AUSTRALIA

Over the next ~150 minutes, there would be a cooldown period,
after which water would flood the mineral ore again and fission would restart.
This three hour cycle would repeat itself for hundreds of thousands of years,
until the ever-decreasing amount of U-235 reached a low-enough level, below
that ~3% amount, that a chain reaction could no longer be sustained. At that
point, all that both U-235 and U-238 could do is radioactively decay.

Looking at the Oklo sites today, we find natural U-235
abundances that range from 0.44% up to 0.60%: all well-below the normal value
of 0.72%. Nuclear fission, in some form or another, is the only
naturally-occurring explanation for this discrepancy. Combined with the xenon,
the neodymium, and the ruthenium evidence, the conclusion that this was a
geologically-created nuclear reactor is all but inescapable.

There are many natural neutrino signatures produced by stars
and other processes in the Universe. For a time, it was thought there would be
a unique and unambiguous signal that comes from reactor antineutrinos. Now we
know, however, that these neutrinos may also be naturally produced. ICECUBE
COLLABORATION / NSF / UNIVERSITY OF WISCONSIN

Interestingly enough, there are a number of scientific
findings we can conclude from looking at the nuclear reactions that occurred
here. We can determine the timescales of the on/off cycles by looking at the
various xenon deposits. The sizes of the uranium veins and the amount that
they've migrated (along with the other materials affected by the reactor) over
the past 1.7 billion years can give us a useful, natural analogue for how to
store and dispose of nuclear waste.

Ludovic Ferrière, curator of the rock collection, holds a
piece of the Oklo reactor in Vienna’s Natural History Museum. A sample of the
Oklo reactor will be displayed permanently in the Vienna museum beginning in
2019. L. GIL/IAEA

The isotope ratios found at the Oklo sites allow us to test
the rate of various nuclear reactions, and determine if they (or the
fundamental constants driving them) have changed over time. Based on this
evidence, we can determine that the rates of nuclear reactions, and therefore
the values of the constants that determine them, were the same 1.7 billion
years ago as they are today.

Finally, we can use the ratios of the various elements to
determine what the age of the Earth is, and what its composition was when it
was created. The lead-isotope and uranium-isotope levels teach us that 5.4
tonnes of fission products were produced, over a 2 million year timespan, in an
Earth that's 4.5 billion years old today. When a supernova goes off, as well as
when neutron stars merge, both U-235 and U-238 are produced.

A supernova remnant not only expels heavy elements created in
the explosion back into the Universe, but the presence of those elements can be
detected from Earth. The ratio of U-235 to U-238 in supernovae is approximately
1.6:1, indicating that Earth was born from largely ancient, not recently,
created raw uranium. NASA / CHANDRA X-RAY OBSERVATORY

From examining supernovae, we know we actually create more
U-235 than U-238 in about a 60/40 ratio. If Earth's uranium were all created
from a single supernova, that supernova would have occurred 6 billion years
before the formation of Earth. On any world, as long as a rich vein of
near-surface uranium ore is produced with greater than 3/97 ratio of U-235 to
U-238, mediated by water, it's eminently plausible for a spontaneous and
natural nuclear reaction to occur.

In one serendipitous location on Earth, in more than a dozen
instances, we have overwhelming evidence for a nuclear history. In the game of
natural energy, don't ever leave nuclear fission off the list again.

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